1. Field of the Invention
The present invention relates to a technology for transmitting information concerning a time reference point from a transmitter to a receiver.
2. Description of the Related Art
In a system that carries out communications and measuring, there are many situations where timing information (corresponding to a time reference point in the present invention) is required to be strictly transmitted and measured at a high speed. For example, in the image communication, there is a situation of synchronizing horizontal and vertical scanning. In the synchronous digital data communication, there is a situation of detecting a timing of changing over bit information. In the measuring, there is a situation of measuring a distance and an angle based on a signal delay time.
In the system that transmits a digitally-modulated information packet, high-speed and highly reliable communications become possible, when a time and an amplitude that become the reference of demodulation can be efficiently and accurately transmitted in plural carrier frequencies.
In the system described above, timing information is transmitted using a wave propagation phenomenon of radio waves that are propagated in space, an optical fiber, a wave guide, or a communication cable, and a wave propagation phenomenon of waves that are propagated in gas, liquid, or a solid.
Assume that a waveform generator, a wave propagation medium, and a waveform receiver that are used in the system have a sufficiently uniform amplitude transmission characteristic and a sufficiently uniform phase transmission characteristic in a wide frequency region. In this case, timing information can be accurately and easily transmitted from the waveform generator to the waveform receiver, by expressing the timing information to be transmitted using a leading edge of a rectangular electric signal pulse or a leading edge of a rectangular ultrasonic burst. Amplitude can be obtained by simply detecting an envelope amplitude.
The electric signal pulse and the ultrasonic burst have a limitless spread in a frequency spectrum. However, the waveform generator, the wave propagation medium, or the waveform receiver actually present has a nonuniform amplitude frequency characteristic and a nonuniform phase frequency characteristic to some extent, and communications are permitted using a frequency of a limited band in many cases. Therefore, a rectangular electric signal pulse and a rectangular ultrasonic burst are deformed at the receiving edge. Particularly, in the communication system that uses a waveform conversion device or a wave propagation medium having a frequency characteristic of a narrow band, and in the remote-distance communication system, it is difficult to strictly transmit timing information, and detection of amplitude is also difficult. Consequently, the information communication speed is limited.
In the field of measuring, there are a radar, a laser measuring device, and an ultrasonic measuring device that measure a distance and an angle based on a signal delay time. Measuring precision of a distance and an angle is directly related to transmission precision (detection precision) of timing information. In other words, the measuring precision of a distance and an angle depends on the transmission precision (detection precision) of timing information. Therefore, in the field of measuring, it is important to accurately (strictly) transmit timing information. For example, there is an ultrasonic distance meter that measures a distance between a transmitter and a receiver by detecting an envelope line leading edge of 40 kHz carrier burst. Distance measuring precision of the ultrasonic distance meter is about a few centimeters to a few tens of centimeters. The ultrasonic distance meter uses a piezoelectric ceramic element for a transmitting and receiving element of an ultrasonic wave. In terms of a speed of wave propagated in the air, the transmission precision of timing information in the ultrasonic distance meter is 0.1 ms to 1 ms. This value is a representative value of precision that can be achieved using an envelope line of a waveform passing through a narrow-band communication path.
The piezoelectric ceramic element used in the ultrasonic distance meter mentioned above has a frequency characteristic of a narrow band. Therefore, the received waveform has a strong distortion. Further, the ultrasonic distance meter uses a region that receives a strong influence of a transient response characteristic of a signal such as a leading edge of the ultrasonic burst. Therefore, a characteristic variation of the transmitting and receiving element easily gives influence to the measuring precision and the transmission precision of timing information. Further, the envelope line of a waveform receives both influences of the amplitude frequency characteristic and the phase frequency characteristic of a transmission path. Therefore, when the transmission path having the above characteristics is used, the shape of the envelope line easily changes, and the transmission precision of timing information has a risk of being decreased as a result.
On the other hand, when timing information can be transmitted using only phase information, transmission precision can be increased from that when the timing information is transmitted using the envelope line. As a result, there is a possibility that measuring and communications can be carried out in higher precision. An angle meter is put into practical use that measures a distance between two points relatively close to each other and that obtains an angle with a signal source from a difference of phases obtained. Further, a micro displacement meter is put into practical use that obtains displacement in a narrow distance, and this micro displacement meter has measuring precision of around 1 mm.
In transmitting timing information using only phase information, a phase needs to be extracted from a cycle phenomenon such as a sinusoidal wave. Because a waveform is repeated in a phase value of 2π radian (cycle in the case of time, and a wavelength in the case of a distance), a phase obtained at the receiving end becomes a surplus obtained after dividing the absolute value of the phase by 2π (remainder after dividing the absolute value by 2π. Accordingly, even when a distance is attempted to be measured by extracting a phase, the absolute value of the distance cannot be measured. Specifically, while the wavelength of an ultrasonic wave of 40 kHz is about 8.5 mm in the air (corresponding to 2π radian of a phase value), only a remainder obtained by dividing the distance by the wavelength of 8.5 mm is obtained and the absolute value of a distance cannot be measured, even when the distance is attempted to be determined by extracting the phase using the ultrasonic wave.
Japanese Patent Application Laid-open No. 2004-191145 is disclosed as an invention according to which an ultrasonic displacement meter using a phase has a measuring range exceeding 8.5 mm corresponding to the wavelength in the air of an ultrasonic wave of 40 kHz. Japanese Patent Application Laid-open No. 2004-191145 discloses a device and a method for obtaining phases φ1 and φ2 by carrying out a phase delay measuring twice by changing over between two different frequencies f1 and f2 (f1>f2), and converting the method to substantially measure a phase using a frequency of “f1-f2” performing various processes based on the obtained phases φ1 and φ2. Specifically, a phase delay is measured by changing over between frequencies of “f1=40 kHz, f2=39 kHz”, thereby obtaining the effect of substantially measuring a phase in the carrier of 1 kHz (40 kHz-39 kHz). As a result, an unstable distance of a displacement meter of 8.5 mm using the ultrasonic wave of 40 kHz can be extended to a wavelength 340 mm of a wave of 1 kHz, while maintaining the measuring precision of around 1 mm.
However, according to the conventional technique, while the measuring distance can be extended to 340 mm, uncertainty of timing information remains due to the phase cycle as the problem of the information transmission technique using a phase. Therefore, although displacement can be measured, the absolute value is difficult to measure.
According to the conventional technique, measuring is carried out at plural times by changing over between frequencies. Therefore, a measuring time becomes long, and a transmission time of timing information becomes long. These problems become particularly noticeable when a propagation medium changes due to a move of an object or a variation of a wind direction and a temperature. In this situation, the measuring itself becomes difficult.